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complete  one, the  more  modem  forms  as usual  being  the more
deficient in numbers of teeth.

The dorso-lumbar vertebrae are as a rule twenty-three; but
the extinct Titanotheres are again an exception; for, at least in
Titanotherium,, there are but twenty of these vertebrae—an Artio-
dactyle character. The femur has a third trochanter. There are
so few recent Peiissodactyles that an enumeration of the dis-
tinguishing characters of the viscera may very probably be use-
less for purposes of classification. But
the living genera at any rate are to be
separated from the living Axtiodactyles
by the invariable simplicity of the
stomach coupled with a very large and
saceulated caecum. The liver is simple
and not much broken up into lobes, and
the gall-bladder is always absent. The
brain is well convoluted. The teats are
in the inguinal region. The placenta in
this group is of the diffused kind.

The living Perissodactyles belong to
three types only, indeed to three genera
only (in the estimation of most), which
are the Horses, Tapirs, and ^Rhinoceroses,
But taking into account the extinct
forms, they may be divided primarily
(according to Professor Osbom) into the
four following groups :—(1) Titano-
therioidea, including but one family,,
Titanotheriidae; (2) Hippoidea, includ-
ing the families Equidae and Palaeotheriidae; (3) Tapiroidea,
with two families, Tapiridae and Lophiodontidae ; and (4) Khino-
cerotoidea with families Hyraeodontidae, Amynodontidae, and
Rhinocerotidae. It is conceivable, according to the same writer,
that the Ghalicotheres (here treated of as a separate sub-order,
Ancylopoda) should be added to the Perissodactyle series.

Fam. 1. Equidae.—This family, which includes the living
Horse, Zebras, and Asses, as well as a number of extinct genera
agreeing with those types in structure, may be defined by the
possession of but one functional toe, the two lateral ones being mere
splints, or but little more. The molax teeth are hypselodjont, and

123.—Anterior aspect of
_,£ht femur of Rhinoceros
(Hhinoceros indicujs}. x •£„
ht Head ; £, great troc&anter;
#", third trochanter. (From
Flower's Osteology.)